Instruments of this general type are used in the course of laparoscopic penetration into, for example, the abdominal cavity. In such penetration, first the abdominal wall is pierced in a suitable area using a thin cannula and then the abdominal space is filled through the cannula with gas.
Thereafter, one or more apertures are created in the abdominal wall above the target of intervention. So-called trocar sleeves are inserted into these apertures. These trocar sleeves are fitted with gas valves and through them the required laparoscopic instruments can be inserted during the intervention into the abdominal cavity.
To assure suitable hermetic sealing, the apertures made in the abdominal wall must have as constant as possible a cross-section over their length. Such apertures allow snug seating of the trocar sleeves with gas-tight sealing.
As a rule, so-called trocars having triangular ground cutting edges at their ends are used to make the apertures. The trocars are pushed through the abdominal wall and the aperture so made is expanded, where called for, by means of dilators.
Disadvantageously, however, the surgeon must exert a relatively large force on the trocar to pierce the abdominal wall. Fine control of the piercing or fine control of the advance is impossible. This may lead to fatal consequences if the surgeon selects a part of the abdominal wall for piercing that may cling to organs.
Some surgeons prefer opening the abdominal wall in the manner of the so-called "mini-laparoscopy" in which a scalpel is made to enter the abdominal wall in short and carefully guided cuts. This technique only calls for slight force, but it has the drawback that the aperture so made is comparatively undefined and thereby may be unsuitable to hermetically seat a trocar sleeve.
Reference is made to the German patent document 34 03 962 C2 which discloses an instrument of the above type to create a tissue canal and which comprises a shaft bearing at its distal end an axially cutting knife. When the shaft is advanced through the tissue, a plane cutting surface of constant width is generated over the entire length of advance and, by laterally displacing the tissue zones adjoining the cutting surface, can be widened into the desired canal without requiring force. However, this instrument also raises the question whether the canal so made can ensure hermeticity for an installed sleeve.